Sensitive, high-precision methods for measuring magnetic signals (arising from magnetic parameters of one or more entities) are of widespread technological interest in fields as diverse as security tagging, targeted drug delivery, non-destructive evaluation in engineering, and geomagnetic surveying. “Magnetic signals” in this context derive from one or more entities, the magnetic parameters (or properties) of which differ significantly from their environment. Typical examples of entities that would possess a set of characteristic magnetic parameters are a soft magnetic security tag embedded in an item of clothing, clinically introduced magnetic nanoparticles in a human body organ, an air-filled void in a metallic matrix and naturally occurring magnetic minerals in rocks.
The most sensitive methods currently used to measure magnetic signals depend on superconducting quantum interference device (SQUID) technology, which imposes design constraints on the method, and places limits on possible applications. In particular, there is the requirement that the SQUID be kept at very low temperatures, such as 4.2 K, the temperature of liquid helium, in the case of “low-temperature” SQUIDs, or 77 K, the temperature of liquid nitrogen, in the case of “high-temperature” SQUIDs. This requirement presents a major design consideration when most of the magnetic signals of interest arise in materials at room temperature or above.
In a typical sensing arrangement, a SQUID and a signal transduction circuit, in the form of a sensor coil coupled to a transfer coil, are arranged within a single cryostat containing liquid helium or liquid nitrogen. The sensor coil is arranged to sense the magnetic field to be measured, and the transfer coil is positioned proximate the SQUID. The current induced in the sensor coil is supplied to the transfer coil, and the magnetic field associated with the current in the transfer coil is sensed by the SQUID. The transfer coil typically has a substantially greater number of turns than the sensor coil, and this provides a means of amplifying the signal.
Such an arrangement provides the advantage of minimising resistance noise in the signal transduction circuit.
However, a major problem in such an arrangement is that, unless the material under investigation is located within the cryostat, the very small magnetic fields being measured must permeate the wall of the cryostat in order to be sensed by the sensor coil. In such cases, the sensor coil is separated from the magnetic signal source by a distance of typically a centimeter or more to accommodate the vacuum and/or radiation shields surrounding the cryogenic liquid, and this necessarily degrades the signal quality. Furthermore, since the cryostat assembly is bulky and not easily manoeuvrable, it is difficult to locate the signals both flexibly and with reasonable accuracy.
It would therefore be desirable to provide an arrangement which can measure extremely low magnetic fields associated with the magnetisation of a material by using a SQUID but without requiring either that the material be located within the cryostat or that the resulting extremely low magnetic fields penetrate the wall of the cryostat containing the SQUID.
Noise considerations have meant that it has generally been regarded as essential that both the sensor coil and the transfer coil be positioned within the cryostat. Such arrangements ensure that the resistance of the coil is extremely low, or even zero in the case of a superconducting coil, thereby reducing noise levels associated with higher-resistance coils. Making the entire transduction circuit superconducting eliminates all thermal (Johnson) noise.